According to the existing communication standards, when a GSM supported mobile is switched ON, it measures the Received Signal Strength Indication (RSSI) of different frequencies in the supported frequency bands and create an ordered list of frequencies from highest RSSI to lowest RSSI and pass it on to a higher layer. Next, the higher layer instructs Layer 1 (L1) to program the radio frequency (RF) module to continuously receive the amplitude/phase (I, Q) sample data and search for the Frequency Correction Channel (FCCH) burst data using an FCCH detector algorithm in the base-band. This is shown in FIG. 1 which is a GSM initial cell selection flow diagram; where FB is the frequency burst of the FCCH, SB is the synchronization burst of the Synchronization Channel (SCH), and BCCH is the Broadcast Control Channel.
Since FCCH burst information carries all zero sequences (e.g. a total of 142+3+3=148 number of zeros), after the Gaussian Minimum Shift Keying (GMSK) modulation at the Base Transceiver Station (BTS), it becomes a pure sine wave tone of frequency Rubidium (Rb)/4=67.77 KHz. The FCCH detector algorithm detects the presence of FCCH information in the received I, Q samples and, if detected, then it provides the frequency error estimated and also the bursts boundaries in time. Knowing these output parameters help to first correct the mobile station (MS) frequency offset. And then, as the burst boundaries are known (burst start position known), so the RF module is programmed for SCH burst reception in the next Time Division Multiple Access (TDMA) frame (since the SCH appears one frame after the FCCH frame in the 51 multi-frame structure as shown in FIG. 2; where F indicates an FCCH frame, S indicates an SCH frame, B indicates a BCCH frame, P indicates a Paging Control Channel (PCCH) frame or a Common Control Channel (CCCH), and I indicates an idle frame. The channels shown in FIG. 2 appear in a time-multiplexed manner in the same frequency and time slot (0) one after the other as shown in FIG. 2.
The SCH contains the Synchronization Burst (SB). The content of the SB structure e.g. data content is shown in FIG. 3. The SB contains, in sequence: 3 tail bits, 39 encoded information bits, 64 Training Sequence (TSC) bits, 39 encoded information bits, 3 tail bits, and 8.25 guard bits. The 64 bits TSC is the same for the whole GSM system and is known to the mobile station (MS) (see the Third Generation Partnership Project (3GPP) Technical Specification (TS) 45.002, section 5.2.5).
The training sequence used by a BTS for the SB burst transmission is (where b indicates the bit number of the SB): b42,b43,b44, . . . , b105=(1,0,1,1,1,0,0,1,0,1,1,0,0,0,1,0,0,0,0,0,0,1,0,0,0,0,0,0,1,1,1,1,0,0,1,0,1,1,0,1,0,1,0,0,0,1,0,1,0,1,1, 1,0,1,1,0,0,0,0,1,1,0,1,1)
The SB burst is always GMSK modulated and sent without any interleaving. That means one burst is enough for decoding. Since the FCCH helps to some extent with obtaining the burst boundary and according to that, the RF window opening time is adjusted to receive the SB burst in the next TDMA frame. Also, coarse frequency error is corrected after the FCCH detection. Once the I,Q samples for the SCH burst are received according to the adjusted RF window, then the I,Q pairs are passed to the base-band for channel estimation, demodulation and decoding of SCH data. The channel estimation process of the SCH is complex since it contains 64 TSC bits. Generally, +−8 bits on both the side of the TSC are allowed to shift and the channel estimation is searched from +−8 bits on both the side of the expected position of the TSC. If the time offset is more than 8 bits, then the channel estimation will fail.
Once the SCH is decoded, it provides 25 information bits. The data includes the Base Station Identity Code (BSIC) and the frame number of the current frame within the hyper frame (19 bit Reduced Frame Number). The BSIC consists of three-bit Network Color Code (NCC) and three bit Base station (BS) color code. This is then used for time synchronization.
The receiver blocks for the FCCH and SCH reception, for FCCH and SCH reception in standard scenarios of MS cell selection, reselection and handover procedures, are shown in the FIG. 4, with FCCH detection block and one SCH detection and decoding block; where RFN indicates Reduced Frame Number.
The above discussed procedure of first receiving FCCH and then SCH in next TDMA frame e.g. after 4.615 ms, is used in many cases, like    (a) During the cell selection and re-selection time: MS detects FCCH and then this is followed by SCH decoding for frequency and time synchronization to the new cell.    (b) During the blind Handover time: MS detects the FCCH and then this is followed by SCH channel decoding for the new handover cell for synchronization.    (c) During neighbor cell monitoring: During the GSM neighbor cell measurement, the MS needs to search the FCCH (which appears in time multiplexed manner in the cell broadcast channel e.g. broadcast frequency and time slot number 0) for all the detected strongest cell frequencies (based on the measured RSSI) in the monitored cell list.    (d) During the periodic cell reconfirmation (BSIC reconfirmation): MS receives and decodes the SCH channel's data for BSIC re-confirmation purpose (see 3GPP TS 45.008).
For the processes of (a) and (b), the MS has to first detect the FCCH and then program the RF module for SCH reception in the next TDMA frame, that means after 8 time slots. This has several potential drawbacks, like:    (1) In I-RAT (Inter Radio Access Technology—when the MS supports many Radio Access Technologies (RATs)) scenario, where the measurement gap time is too short, whereby performing the FCCH and SCH reception in a single time gap (which is provided by the other, active, RAT to the GSM RAT) is difficult, and also aligning the FCCH and SCH frames with the gap is a problem. As shown in the below FIG. 5, FCCH detection is attempted in many gaps and finally succeeded in the Gap#N. SCH decoding is attempted next, but since the SCH slot is not aligned with a gap elsewhere than in Gap#M, the SCH is decoded with a large latency.    (2) In the dedicated mode, the measurement time available is very short and precious. In a Long Term Evolution (LTE) active RAT case, the gaps for measurements on a GSM RAT are not regular since LTE has Discontinuous Reception (DRX) in connected mode. So, also in that situation, making sure that all the neighbor cells' SCH will fall under the time gap period is very difficult and not possible to achieve (as illustrated in FIG. 6, where C indicates the Common Control Channel (CCCH) which e.g. is PCCH or Scheduling Grant Channel (SGCH)). Since we have a two-step based approach as shown in FIG. 5 (FCCH followed by SCH), Cell selection/re-selection takes more time.    (3) In Blind Handover it takes a lot of time to search for FCCH and SCH, as shown in FIG. 1.    (4) In periodic cell reconfirmation it takes a lot of extra processing cycles to reconfirm the cell by searching for FCCH and then SCH.
Regarding cell reconfirmation, after the camp on when MS is in idle mode, the MS shall continue to monitor all BCCH carriers as indicated in the BCCH Allocation (BA) list [3GPP TS 45.008 section 6.6.1]. The MS shall first monitor the RSSI of the non-serving (neighboring) carriers (up to 32 carriers). Then, if a new carrier is found whose signal strength is greater than a defined threshold, the MS will schedule for FCCH detection on that carrier and if FCCH is found, next SCH detection will be scheduled by the MS for that carrier after getting the rough timing information about in which TDMA frame the SCH on that carrier will be appearing. If the SCH is decoded successfully, then the decoded SCH data will convey the RFN (reduced frame number which is 19 bits) and BSIC (base station identity code BCC and network identity code NCC). The MS checks for the validity of the BSIC and if the BSIC is new e.g. the cell is new and allowed then the cell is added in the cell list if it has not been added earlier e.g. if it is newly found cell. Once a new cell is found and added in the cell list, the MS tries to monitor that cell on a regular basis to re-confirm that it is monitoring the same cell. This process is known as BSIC reconfirmation. Also in the dedicated mode, the MS does the BSIC reconfirmation.
According 3GPP TS 45.008, the MS shall attempt to check the BSIC for each of the 6 strongest non serving cell BCCH carriers at least every 30 seconds in idle mode, to confirm that it is monitoring the same cells. According to TS 45.008 section 7.2, it is essential for the MS to identify which surrounding Base Station Subsystem (BSS) is being measured in order to ensure reliable handover. Thus, it is necessary for the MS to synchronize to and demodulate surrounding BCCH carriers and identify their base station identification codes (BSIC). The MS shall attempt to demodulate the SCH on the BCCH carriers of as many surrounding cells as possible, and decode the BSIC as often as possible, and as a minimum at least once every 10 seconds in connected mode.
A multi-RAT MS is allowed to extend this period to 13 seconds in connected multi-RAT scenarios, if the neighbor cell list contains cells or frequencies from other RATs. In a multi-RAT scenario, an MS shall thus, for a period of up to 5 seconds, devote all search frames to attempting to decode these BSICs. If this fails then the MS shall return to confirming existing BSICs. Having re confirmed existing BSICs, if there are still BCCH carriers among the six strongest with unknown BSICs, then the decoding of these shall again be given priority for a further period of up to 5 seconds. In Packet Idle mode or MAC-Idle state (TS 45.008, section 10.1.1.1) the MS shall attempt to check the BSIC for each of the 6 strongest non serving cell BCCH carriers at least every 14 consecutive paging blocks of that MS or 10 seconds, whichever is greater. If a change of BSIC is detected then the carrier shall be treated as a new carrier, else continue with the same carrier.
The same is applicable for Packet Transfer mode or MAC-Shared state (TS 45.008, section 10.1.1.2), where an MS shall continuously monitor all BCCH carriers as indicated by the BA (General Packet Radio Services (GPRS)) list and the BCCH carrier of the serving cell. A list containing BSIC and timing information for these strongest carriers at the accuracy required for accessing a cell (see 3GPP TS 45.010) including the absolute times derived from the parameters T1, T2, T3 shall be kept by the MS. This information may be used to schedule the decoding of BSIC and shall be used when re-selecting a new cell in order to keep the switching time at a minimum. When a BCCH carrier is found to be no longer among the reported, BSIC and timing information shall be retained for 10 seconds.
As mentioned above, the BSIC identification happens very frequently in the MS and happens in idle as well as in dedicated mode or packet transfer mode. For a single RAT GSM/GPRS MS, it is a high burden to demodulate and decode the SCH burst all the time for all the detected cells for reconfirmation purpose. Because it consumes a lot of battery power for SCH burst equalization, demodulation and decoding purposes and at the same time it consumes some time for decoding (though it is a single burst decoding).
In a Multi-RAT MS, where the MS needs to do many things in a small provided time gap (given from other RATs to GSM), the time for decoding SCH data matters a lot for the overall MS performance. The reconfirmation process discussed above is illustrated in FIG. 7; where C1, C2, . . . Cn indicate the strongest GSM cells.